The majority of “controlled-release” drug delivery systems known in the prior art (e.g., U.S. Pat. No. 4,145,410 to Sears which describes drug release from capsules which are enzymatically labile) are incapable of releasing drugs at intervals and concentrations which are in direct proportion to the amount of a molecular indicator (e.g., a metabolite) present in the human body. The delivery or release of drug in these prior art systems is thus not literally “controlled,” but simply a slow release which is independent of external or internal factors.
The treatment of diabetes mellitus with injectable insulin is a well-known and studied example where uncontrolled, slow release of insulin is undesirable. In fact, it is apparent that the simple replacement of the hormone is not sufficient to prevent the pathological sequelae associated with this disease. The development of these sequelae is believed to reflect an inability to provide exogenous insulin proportional to varying blood glucose concentrations experienced by the patient. To solve this problem several biological and bioengineering approaches to develop a more physiological insulin delivery system have been suggested (e.g., see U.S. Pat. No. 4,348,387 to Brownlee et al.; U.S. Pat. Nos. 5,830,506, 5,902,603, and 6,410,053 to Taylor et al. and U.S. Patent Application Publication No. 2004-0202719 to Zion et al.).
In certain embodiments of the Zion system, multivalent glucose-binding molecules (e.g., lectins) are combined with a glycosylated polymer-insulin conjugate. The glycosylated polymer contains multiple saccharide binding groups and forms an insoluble cross-linked material in the presence of the glucose-binding molecule. The material releases the glycosylated polymer-insulin conjugate in response to increases in glucose concentration. In general, these systems have so far relied on high molecular weight carbohydrate structures that are based on natural carbohydrates such as dextran and glycogen. As discussed below, while these high molecular weight natural carbohydrates are useful, they present certain difficulties and there is therefore a need in the art for alternative conjugates with novel properties and functionalities.